How to Slate a Roof

How to install a slate roof

Slating a roof can be quite a daunting task, it’s one of those things that appear very easy until you look into it in a bit more detail, then all of a sudden it can become very complicated. This is certainly true when lots of technical phrases are used on the uninitiated. I’m going to try and give you in this article, and with the aid of a video or two, enough information for you not only to complete your own simple slate roof, but to feel confident enough to research larger more complicated jobs.

But before we get all technical, please watch this video, It’s designed to give a basic insight into laying out a slate roof, including the first starter course or eaves row of slates. How to slate a roof – This video shows some of the considerations you should make when planning your slate roof including how to lay the slates onto the roof, and how to fit the battens to gauge.

The above video also shows how to adjust batten gauge (batten spaces) by adding an extra batten if needed which will increase headlap, but please make sure before fully battening your roof that you can still nail your slates without needing to re-hole them if you are using pre holed slates. Depending on the height of the roof, size of slates and size of the battens it may or may not be possible to simply alter your nailing position height onto the battens to compensate, especially if it’s a low height roof using very large slates. I simply cannot fit all the possible scenarios into a 15 minute video and make it make sense, slating is a skilled job.

How to lay a slate roof – This video takes you through the process of laying slate onto the roof and includes details like abutting walls, verges, gables, eaves slates (starter course) and top slates.

Make a batten gauge block

After you have worked out your desired batten gauge, consider making a batten gauge block from an offcut of batten. When placed end up on a rafter it will allow quick and easy spacing between roof battens for every batten cut and fixed above it. Always check that spacings are still correct and everything is parallel a few courses before the top of the roof to apply fine corrections though.

A gauge block – Shown here placed between the battens, it will allow quick and accurate positioning for all the subsequent battens above it.

How to Install a slate roof – The details

Here are some of the things that are covered in more detail…

Roof pitch – The angle of a roof not only determines what slates to use, but the amount of overlap that the slate roof needs to remain waterproof.

Slating terms and phrases – If you are not a seasoned roofer, some of the terms used for slates, batten spacing or overlapping of the slates can add to the confusion. If you can understand the anatomy of a slate and the words used for the different bits, you will be a lot clearer in your mind.

Roof battens and gauge – The gauge is the space between the roof battens, this will set the overlap of your chosen slate in most cases.

What roof pitch do I need for what slate size

Generally speaking 20 degrees is the lowest advisable pitch for a slate roof, depending on the size of the slates and how much that the roof is exposed to moderate weather conditions. When slating with a shallow pitch a wide slate is highly recommended for maximum side lap or lateral lap (side overlaps). This usually means a large artificial slate or man made slate like Eternit or similar, but can also include some large natural slates.

Slate roof calculator

Be very careful if attempting to calculate the amount of slate you require for your roof, square metreage, slate size, and headlap or gauge are all factors that can affect the amount of slate needed for a roof. I would advise asking your roofing supplier or slate merchant, normally if you give them the amount of surface area in square metres, your slate size and overlap, they will do all the maths for you, including breakage allowance.

How much headlap for slates

What is headlap? See Glossary further down the page

Before we see which slates are best for a given degree of slope, or discuss the amount of headlap (overlap) required you also have to think of exposure to the elements. There are diagrams for geographical regions as well as recommended headlap’s for most slates, but just before you follow them blindly, take into account any other weather or climate conditions that make your proposed slate roof more exposed than it otherwise should be. This could for instance be a three or four story house, on top of a hill, facing in the direction of strong winds or being discharged onto by a flat roof or guttering downpipes. All of these things are especially important if you are considering a slate roof below a 40 degree pitch.

Minimum slate headlap – Moderate weather – This table of slate overlaps and roof angles shows common sizes of roofing slate and the recommended slope for the corresponding slate size. If your property’s roof is in the green moderate part of the UK but you know of prevailing bad winds or weather, you may wish to use the other chart instead.

Minimum slate headlap – Severe weather – This chart shows slate headlap’s for roof slates that are exposed to bad weather conditions. Only consider reducing the slate overlaps if you know that your roof is in a sheltered location, but be very careful on shallow roofs.

Slating terms and phrases

Slates – A typical slate has a top, bottom and two sides. The top of the slate is known as the ‘ head ‘ and the bottom the ‘ tail ‘ , the head sits on top of the roofing batten and the tail sits on top of the slate beneath it. These days a lot of slates both natural and artificial are factory made and are very uniform in size and thickness, if the slate is not a uniform thickness the head is normally the thinner end and the tail the thicker. The front of a slate will display riven (bevelled) edges on all sides and this is called the ‘ face ‘ , and the underneath is the ‘ bed ‘.

Slating diagram – Here is the anatomy of your average slate roof including some of the common phrases used. If you understand the phrases and what parts of the slate or roof they refer to it helps a great deal.

Please use this slating diagram to refer back to.

Nail holes – The nail holes in a slate are normally punched 20 to 25mm (3/4” to 1″) in from the outside edges of the slate, unless the slate is fixed via the head (top) with a nail or a wooden peg. Most new slate is either pre-holed or can be requested to be pre-holed when ordering, and nearly all reclaimed slates have been holed already. Be very careful when considering re-holing the slates further in form the sides than the normal 20 – 25mm as this places the holes closer the centre of the slate and can be a bigger cause of roof leaks than incorrect headlap. (overlap)

Double lap slates – The most common type of slate roof in the U.K. It basically means that every slate is overlapped twice. This can also be seen and described in my first video on how to slate a roof at the top of the page. The other alternative is triple lap, but is quite rare, basically the gauge or battens are closer together so each slate is overlapped three times.

Headlap or Lap – As discussed earlier, the top of a slate is called the ‘ head ‘ and when a slate above overlaps this as part of the water proofing process it creates an overlap or head lap, so headlap is the name given to this so it’s easy to refer to. Headlap or recommended headlap is often specified for a given roof pitch, otherwise known as a roof slope or roof angle.

Our first look at headlap comes in rows 1 and 2 where the top slate in the picture overlaps the one underneath (row 0 being the cut height eaves slate). This head lap will be repeated all the way up the roof. Headlap can also be specified by weather conditions, please see the roof pitch and slate choice section on this page. Headlap these days is normally between 75 and 100mm (3″ to 4″)

Side lap – Side lap, Bond, or half bond is where one slate overlaps another, as one course goes on top of the other, a slate is placed straddling the two slates beneath in a brick bond pattern. These overlaps are called side lap which should be no less than 75mm (3″) ideally. (see diagram above or main slating diagram)

Side lap gap – The gap between abutting slates should be between 1 – 5mm, most people these days quote 3mm, up to a maximum of 9mm for fibre cement slates. There is normally enough wiggle room here to close or open the gap from 3mm, to space out slates in their horizontal axis when first laying out a slate roof for adjustment purposes.

Margin – This is the visible or exposed area of a slate from the bottom or ‘ tail ‘ of one slate up to the point the slate above overlaps it. Another name for the margin is the ‘ exposure ‘ . If you measure the margin it will be the same measurement as the Gauge.

Gauge – This is the distance measured between the top of one slate and the top of another slate laid underneath or above it, which is normally determined by the height of the nail holes in the slate, and will be the same distance as the margin and batten gauge in most cases.

Slate gauge -It doesn’t matter whether you measure gauge, from top to top or bottom to bottom, on a roof with slates of the same size the measurement will always be the same.

The height of the hole punched into a slate will set the headlap for the slate in most cases, however some tweaking is possible by altering the height of where you nail the slate onto the batten in some circumstances, and I’ll show you this later on. However to work out the gauge and batten gauge for any slate either pre-holed or not, you can use this simple math…

Height – lap / 2 = Gauge… lets view this another way…

Height of slate (minus) desired headlap (divided by two) = Gauge

Say a slate is 600mm long (24″) and you wanted a headlap of 75mm (3″) then the maths would be 600mm – 75mm divided by two = a Gauge of 263mm

Batten gauge – Another phrase for batten spacing, this is normally the same as the Gauge mentioned above, but is the distance measured from the top of a batten, to the top of the batten above that. The recommended batten gauge for any slate can be worked out in the same way as the gauge because the top of the slates sit in the same position on the batten with every row, so they share the same gauge.

Adjusting batten gauge

Batten gauge can sometimes be altered however to adjust headlaps, or change the spacing between slate courses so the top area of visible slates on a roof look neat. This is normally done by adding another batten which reduces batten gauge and increases headlap, or by stretching, which is increasing batten gauge slightly. This is always done in accordance with the recommended headlaps however.

Changing batten gauge – This picture better shows how shrinking or stretching a recommended batten gauge and fixing position on the batten will alter both distance, (for example to the top of a roof) as well as headlap. Always make sure you keep the minimum headlap you required. The pictured slates are exactly the same, only batten gauge and nailing position change.

Universal holing gauge

Batten gauge can also be adjusted to suit a range of ” universal “ holing gauge slates that are more common these days, by altering the batten gauge in relation to the pre-set holing gauge, the headlap is increased or decreased without re holing simply by changing the fixing position of the slates onto the batten. By having a universal holing gauge the slate manufacturer can save time and money by supplying slates factory pre holed to suit more customers, you will often get suggested headlaps, batten gauges, roof pitch and exposure recommendations too.

Universal holing gauge – This is what Cembrit have to say about the universal holing gauge on their slates.

Holing gauge – This is measured from the bottom of a slate ‘ tail ‘ to the nail hole. Should you need to re-hole slates, firstly we need to know the gauge, calculate this by using the maths below.

Height of slate (minus) desired headlap (divided by two) = Gauge i.e. for a 500mm long slate with a 100mm headlap the gauge is 500 – 100 = 400 / 2 = 200 so we know the gauge is 200mm

Now we can work out the holing gauge with Gauge + Headlap + X

X = The nail hole clearance, so that a nail does not strike the top of the slate underneath. The best place being 3 – 5mm as this gives the best resistance to wind uplift, so for our example we will work with 4mm although it can be as high as 15mm in some cases.

Gauge + Headlap + 4 i.e. 200 + 100 + 4 = 304 so we know the holing gauge would be 304mm from the bottom of the slate.

Diminishing batten gauge – This is normally found on graded slates, where slates vary in size, not only in height but in width too. Very common with vernacular slate (slates from local sources) like Swithland slate. The slates have to be graded from the thickest and widest that sit on the bottom courses, through mid size and ending with the smallest slates at the top of the roof. You can work out the gauge and batten gauge for every course if necessary by using the equation above in the ‘ Gauge ‘ section.

Diminishing Gauge – There is quite a lot of local slate or stone that uses a batten gauge that reduces like this. This is in ‘ Tintagel ‘ in Cornwall for those who might be interested.

Another place to find a diminished gauge can be where a roofer adjusts his battens slightly half way up a roof for instance, this is normally not noticeable to the naked eye and is usually to sit the top row of slates neatly under ridge tiles or lead flashings. This is sometimes done in the same manner as ” universal ” holing gauge slates, by altering the nail fixing position into the laths.

Roof battens or Slating lath

Two different terms for the same thing, tanalised timber battens that are nailed to the roof rafters or roof trusses. These are nailed horizontally at regular spaces depending on the slate size you have chosen and the amount of slate overlap you require. Three common sizes immediately spring to mind.

25mm x 38mm (1″ x 11/2 “) Commonly referred to as tiling lath or tiling batten, but also used for slating. Now often used for rafter spans of 450mm or less.

25 or 30mm x 50mm (1 – 11/4 ” x 2″) Mostly used in newer spec tiled roof’s but now gaining popularity for new slate roofs and often used for rafter spans of 450 – 600mm. The wider batten also allows for greater adjustment to nailing positions as used in universal gauge.

Slate batten choices – Over the years slating battens have got larger, it does increase the cost but they are easier to work with and safer to work on. All battens should be fixed into the rafters or trusses underneath by nails with a minimum of 40mm or 11/2” of penetration into the rafters.

The 3 4 5 square

On the side issue of roofing battens or laths, if you to need the check for square or mark battens to ensure slates keep their correct spacings and remain in line, one of the easiest ways is to build a builders square or carpenters square. Simply nail together 3 lengths of roofing lath so that for instance one length is 3 feet, the next one four feet and the last one five feet, this will make a triangle with two square edges to work with.

The 345 rule – Thanks to Mr Pythagoras and Pythagorean Theorem, making a square is very easy to do with timber. The units can be feet or meters, upscaled or downscaled as required as long as the proportions are constant.

Want to know more?

Another great source of roofing related knowledge I have stumbled across is a great little book called Roof Tiling and Slating and having read it I can definitely recommend it. Whilst it I found it skipped over Gauge adjustment slightly for slate height finish, it did at least get a mention which deserves credit.

It also lists tools, materials, roof shapes and considerations for planning your job. Slates, both natural and man made, as well as interlocking and plain tiles, there’s also information on valleys, ridges, verges and other details I have yet to cover in videos or on the website so far.